Heat engines are used to convert thermal energy into useful mechanical work and are often used in power generation plants. One common example of a heat engine is an expander-generator system, which generally includes an expander (e.g., a turbine) rotatably coupled to a generator or other power generating device. As process fluids are expanded in the expander, the shaft connecting the turbine and generator rotates and generates electricity in the generator.
Many power plant expander-generators are based on the Rankine cycle and obtain high-temperature, high-pressure process fluids to expand by pumping a fluid in a pump and evaporating and heating the fluid via combustion of coal, natural gas, oil, and/or nuclear fission. Common fluids for such engines include water and air. Recently, however, due to perceived benefits in terms of hardware compactness, efficiency, heat transfer characteristics, etc., there has been considerable interest in using super-critical carbon dioxide (ScCO2) as a process fluid for certain expander-generator applications. Notable among such applications are nuclear, solar, and waste heat energy conversion cycles. A challenge to implementing practical waste heat recovery systems using ScCO2 is that such systems often create a problematic combination of relatively high-pressure and high-temperature process fluids that are difficult to effectively contain.
One common solution to handle the high-pressure, high-temperature fluids is installing the expander flowpath components in an un-split barrel casing. In a typical barrel casing configuration, the internal components are aligned with each other, both axially and radially, by concentric circumferential fits against the inner surface of the barrel casing. This solution is effective for high-pressure applications, but generally only at modest temperatures (e.g., below 600° F.). In higher-temperature applications, the use of such casings can still allow temperature-sensitive components of the machines to be exposed to temperatures above their safe operating ranges for extended periods of time, which can lead to component failure. For example, dry gas seals, elastomeric seals, and carbon ring seals may be capable of withstanding the pressures in the machine, but may be ill-suited for such high temperatures.
What is needed, therefore, is an apparatus and method for controlling temperatures in high-temperature during expansion while maintaining efficient sealing and precise alignment.